U.S. patent number 6,625,357 [Application Number 09/982,382] was granted by the patent office on 2003-09-23 for method for fabricating fiducials for passive alignment of opto-electronic devices.
This patent grant is currently assigned to Tyco Electronics Corporation. Invention is credited to Richard Anderson, Terry Patrick Bowen, John Baker Breedis, Ching-Long Jiang, William Sean Ring, Mark S. Soler, Randall B. Wilson.
United States Patent |
6,625,357 |
Bowen , et al. |
September 23, 2003 |
Method for fabricating fiducials for passive alignment of
opto-electronic devices
Abstract
The present invention relates to a technique for fabricating a
mechanical or visual alignment fiducial on a laser die particularly
adapted for application with a laser die that is a buried structure
edge emitting laser. In fabricating the device, the fiducial and
the active mesa are formed in the same photolithography patterning
step, using conventional techniques. The active is then buried with
regrowth layers. The regrowth layers are subsequently selectively
etched to expose the fiducial, leaving the active region protected
and buried.
Inventors: |
Bowen; Terry Patrick (Etters,
PA), Ring; William Sean (Ringoes, NJ), Jiang;
Ching-Long (Belle Mead, NJ), Wilson; Randall B. (Warren,
NJ), Soler; Mark S. (Whippany, NJ), Breedis; John
Baker (Boston, MA), Anderson; Richard (N. Attleborough,
MA) |
Assignee: |
Tyco Electronics Corporation
(Middletown, PA)
|
Family
ID: |
31996841 |
Appl.
No.: |
09/982,382 |
Filed: |
October 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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277909 |
Mar 29, 1999 |
|
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Current U.S.
Class: |
385/49; 385/88;
438/39 |
Current CPC
Class: |
G02B
6/4228 (20130101); G02B 6/3636 (20130101); B82Y
20/00 (20130101); H01S 5/227 (20130101); H01S
5/34306 (20130101); G02B 6/3652 (20130101); H01S
5/0237 (20210101); H01S 5/02375 (20210101); H01S
5/02251 (20210101); H01S 5/02326 (20210101); H01S
2304/04 (20130101); H01S 5/2224 (20130101); H01S
5/3434 (20130101); G02B 6/3692 (20130101) |
Current International
Class: |
G02B
6/30 (20060101); H01L 21/00 (20060101); G02B
006/30 () |
Field of
Search: |
;430/22,321,311
;385/90,88 ;372/50 ;356/401 ;438/49 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wille; Douglas A.
Parent Case Text
RELATED APPLICATION
The present application is a continuation-in-part of U.S. patent
application Ser. No. 09/277,909, filed Mar. 29, 1999, now
abandoned, which is incorporated herein by reference.
Claims
We claim:
1. A method for fabricating a light generating die with fiducials
from a multi-layered structure comprising a substrate and an
etch-stop layer, the method comprising the steps of: (1) depositing
an etch-stop layer over said structure; (2) etching through said
etch-stop layer and said structure to form an active mesa and a
fiducial, said fiducial comprising at least one feature positioned
in a known spatial relation relative to said mesa; (3) regrowing
one or more layers on said structure; and (4) selectively etching a
portion of said regrowth layer to expose said fiducial.
2. The method of claim 1 wherein said fiducial is a mechanical
fiducial comprising a second mesa defining an alignment notch
having a first surface along a top surface of said etch-stop layer
of said second mesa and a second surface along a side surface of
said second mesa.
3. A method for mounting said die of claim 2 to a substrate, said
method further comprising the steps of: (5) placing said die on
said substrate; (6) moving said die to cause said second surface to
abut a mating surface on said die, said mating surface positioned
so as to cause said die to be in a desired position on said
substrate in at least a first direction; and (7) attaching said die
to said substrate.
4. The method of claim 3 wherein step (6) comprises moving said die
on said substrate so that said first surface of said die registers
with a mating surface on said substrate, said mating surface
positioned so as to cause said die to be in a desired position on
said substrate in at least a second direction.
5. The method of claim 1 wherein said fiducial is a visual fiducial
comprising a void in said multilayer structure having visible
features in at least two dimensions.
6. A method for mounting a die fabricated as set forth in claim 5
to a substrate, said method further comprising the steps of: (8)
optically detecting said visual fiducial on said die; (9) optically
detecting a mating visual fiducial on said substrate positioned on
said substrate such that, when said visual fiducial on said die
overlays said mating visual fiducial on said substrate, said die is
in a desired position on said substrate; (10) optically aligning
said fiducial on said die with said mating fiducial on said
substrate so that they overlay each other; and (11) attaching said
die to said substrate.
7. The method of claim 5 further comprising the step of: (12)
forming metal pads on said die using said visual fiducial as an
optical indicia for positioning of said metal pads on said die.
8. The method of claim 7 further comprising the step of: (13)
placing a wettable material on said metal pads.
9. A method for mounting a die fabricated as set forth in claim 7
to a substrate having pads positioned to mate with said pads on
said die, said method further comprising the steps of: (14) forming
a wettable material on at least one of said pads on said die and
said pads on said substrate; (15) placing said die on said
substrate such that each said pad on said die roughly overlays said
mating pad on said substrate; and (16) melting said wettable
material, whereby said die is brought into fine alignment with said
mating pad on said substrate via surface tension of said wettable
material.
10. The method of claim 5 wherein said visual fiducial comprises at
least two distinct visual fiducials, each having visible features
in at least two dimensions.
11. The method of claim 10 further comprising the step of: (17)
placing a wettable material in said voids on said die.
12. The method of claim 10 further comprising the steps of: (18)
forming metal pads in said voids.
13. The method of claim 11 wherein step (17) comprises forming
metal pads in said voids and subsequently placing said wettable
material on said metal pads.
14. A method for mounting a die fabricated as set forth in claim 11
to a substrate having pads positioned to mate with said voids on
said die, said method further comprising the steps of: (19) placing
said die on said substrate such that each said void on said die
roughly overlays said mating pad on said substrate; and (20)
melting said wettable material, whereby each of said pads on said
die is brought into fine alignment with said mating pad on said
substrate via surface tension of said wettable material.
15. A method for mounting a die fabricated as set forth in claim 12
to a substrate having pads positioned to mate with said voids on
said die and a wettable material on said pads, said method further
comprising the steps of: (21) placing said die on said substrate
such that each said void on said die roughly overlays said mating
pad on said substrate; and (22) melting said wettable material,
whereby each of said pads on said die are brought into fine
alignment with said mating pad on said substrate via surface
tension of said wettable material.
16. A method for mounting a die fabricated as set forth in claim 13
to a substrate having metal pads positioned to mate with said voids
on said die, said method further comprising the steps of: (23)
placing said die on said substrate such that each said void on said
die roughly overlays said mating metal pad on said substrate; and
(24) melting said wettable material, whereby each of said metal
pads on said die is brought into fine alignment with said mating
metal pad on said substrate via surface tension of said wettable
material.
17. The method of claim 1, wherein step (2) comprises reactive ion
etching.
18. The method of claim 1, wherein step (2) comprises depositing an
etch mask over said etch-stop layer, said etch mask defining said
active mesa and said fiducial.
19. The method of claim 17, further comprising the steps of, after
step (2): (25) disposing a photoresist over said fiducial; (26)
further etching said structure; and (27) removing said etch mask
and said photoresist to expose said fiducial.
20. The method of claim 19, wherein step (26) comprises further
etching said active mesa.
21. The method of claim 20, wherein said active mesa is defined at
least partially by etched cavities surrounding said active mesa,
said method further comprising the step of: (28) disposing a
blocking layer in the etched cavities defining said active
mesa.
22. The method of claim 21 wherein step (3) comprises at least
disposing protective layers on said fiducial.
23. The method of claim 22, wherein step (3) comprises disposing a
burying layer on said active mesa.
24. The method of claim 23, wherein said blocking layer is disposed
by MOCVD or LPE.
25. The method of claim 23, further comprising the steps of, prior
to step (1): (29) providing a buffer layer on said substrate; (30)
growing a first quaternary layer on said buffer layer; and (31)
growing a cladding layer on said first quaternary layer, wherein
said layers created in steps (29)-(30) comprise said multilayer
structure.
26. The method of claim 25 wherein step (1) comprises growing a
second quaternary layer on said cladding layer, said second
quaternary layer being said etch-stop layer.
27. The method of claim 1, further comprising disposing a blocking
layer between said central mesa and said side mesa.
28. The method of claim 1, wherein said protective layer comprises
blocking material.
29. The method of claim 27, wherein said growing step comprises
disposing a burying layer on a top surface of said etch-stop layer
of said central mesa and on a top surface of said protective
layer.
30. The method of claim 27, wherein said disposing of said blocking
layer is by MOCVD.
31. The method of claim 27, wherein said disposing of said blocking
layer is by LPE.
32. The method of claim 1, wherein said disposing of said
protective layer is by MOCVD.
33. The method of claim 1, wherein said disposing of said
protective layer is by LPE.
34. A method for fabricating a buried heterostructure edge-emitting
laser with a fiducial from a multi-layered structure comprising a
substrate and an etch-stop layer, said method comprising the steps
of: (1) creating a multi-layered structure on a substrate; (2)
growing a first quaternary layer on said multi-layered structure,
said quaternary layer being an etch-stop layer; (3) etching through
said etch-stop layer, cladding layer, first quaternary layer, and
buffer layer to form an active mesa and a fiducial, said fiducial
comprising at least one surface positioned in a known spatial
relation relative to said mesa; (4) disposing a photoresist over
said fiducial; (5) further etching said active mesa; and (6)
removing said etch mask and said photoresist to expose said
fiducial and define etched cavities surrounding said active mesa;
(7) disposing a first blocking layer in the etched cavities
defining said active mesa and a second blocking layer over said
fiducial; (8) disposing a burying layer over said active mesa, said
blocking layer, and said fiducial; and (9) selectively etching said
burying layer and said second blocking layer to expose said
fiducial.
35. The method of claim 34 wherein step (1) comprises the steps of:
(1.1) providing a buffer layer on said substrate; (1.2) growing a
second quaternary layer on said buffer layer; and (1.3) growing a
cladding layer on said second quaternary layer;
36. The method of claim 35, wherein said blocking layer is disposed
by MOCVD or LPE.
37. The method of claim 36, wherein step (3) comprises reactive ion
etching.
38. The method of claim 36, wherein step (3) comprises depositing
an etch mask over said etch-stop layer, said etch mask defining
said active mesa and said fiducial.
39. The method of claim 36 wherein said fiducial is a visual
fiducial comprising a void in said multilayer structure.
40. The method of claim 39 further comprising the step of: (10)
placing wettable material in said voids on said die.
41. The method of claim 40 wherein step (10) comprises forming
metal pads in said apertures and subsequently placing said wettable
material on said metal pads in said voids.
42. The method of claim 39 further comprising the step of: (11)
forming metal pads in said voids on said die.
43. A method of mounting a die fabricated as set forth in claim 40
to a substrate having metal pads positioned to mate with said voids
on said die, said method further comprising the steps of: (12)
placing said die on said substrate such that each said void on said
die roughly overlays said mating metal pad on said substrate; and
(13) melting said wettable material, whereby each of said metal
pads on said die are brought into fine alignment with said mating
metal pad on said substrate via surface tension of said wettable
material.
44. A method of mounting a die fabricated as set forth in claim 42
to a substrate having metal pads positioned to mate with said voids
on said die, said method further comprising the steps of: (14)
placing a wettable material on one of said pads on said substrate
and said pads on said die; (15) placing said die on said substrate
such that each said void on said die roughly overlays said mating
metal pad on said substrate; and (16) melting said wettable
material, whereby each of said metal pads on said die are brought
into fine alignment with said mating metal pad on said substrate
via surface tension of said wettable material.
Description
FIELD OF THE INVENTION
The present invention relates to a technique for fabricating
fiducials in a buried heterostructure edge emitting laser for
alignment of the device in a passive manner.
BACKGROUND OF THE INVENTION
The present invention is related to U.S. Pat. No. 5,981,975 to
Imhoff, filed Feb. 27, 1998 as well as to U.S. patent application
Ser. No. 60/079,910 filed on Mar. 30, 1998, the disclosures of
which are specifically incorporated herein by reference. Light
emitting devices often utilize double heterostructures or
multi-quantum well structures in which an active region of a III-V
semiconductor is sandwiched between two oppositely doped III-IV
compounds. By choosing appropriate materials for the outer layers,
the band gaps are made to be larger than that of the active layer.
This procedure, well known to one of ordinary skill in the art,
produces a device that permits light emission due to recombination
in the active region, but prevents the flow of electrons or holes
between the active layer and the higher band gap sandwiching layers
due to the differences between the conduction band energies and the
valence band energies, respectively. Light emitting devices can be
fabricated to emit from the edge of the active layer, or from the
surface. Typically, a first layer of material, the substrate, is
n-type indium phosphide (InP) with an n-type buffer layer disposed
thereon. This buffer layer again is preferably InP. The active
layer is typically indium gallium arsenide phosphide (InGaAsP) with
a p-type cladding layer of InP disposed thereon. One potential
pitfall of double heterostructure lasers is often a lack of means
for confining the current and the radiation emission in the lateral
direction. The result is that a typical broad area laser can
support more than one transverse mode, resulting in unacceptable
mode hopping as well as spatial and temporal instabilities. To
overcome these problems, modern semiconductor lasers employ some
form of transverse optical and carrier confinement. A typical
structure to effect lateral confinement is the buried
heterostructure laser. The buffer, active and cladding layers are
disposed on the substrate by epitaxial techniques. The structure is
then etched through a mask down to the substrate level leaving a
relatively narrow (roughly on the order of several microns)
rectangular mesa composed of the original layers. A burying layer
is then regrown on either side of the mesa resulting in the buried
heterostructure device. The important feature of a buried
heterostructure laser is that the active layer is surrounded on all
sides by a lower index material so that from an electromagnetic
perspective the structure is that of a rectangular dielectric
waveguide. The lateral and transverse dimensions of the active
region and the index discontinuities are chosen so that only the
lowest order transverse mode can propagate in the waveguide.
Another very important feature of the structure and that which is
required to effect lasing is the confinement of injected carriers
at the boundaries of the active region due to the energy band
discontinuities at the interface of the active region and the InP
layers. These act as potential barriers inhibiting carrier escape
out of the active region.
One area of optoelectronics which has seen a great deal of activity
in the recent past is in the area of passive alignment. Silicon
waferboard, which utilizes the crystalline properties of silicon
for aligning optical fibers, as well as passive and active optical
devices, has gained a great deal of acceptance. One technique for
aligning an optoelectronic device to an optical fiber and other
passive and/or active elements is the use of an alignment pedestal
for lateral planar registration and standoffs for height
registration. By virtue of the sub-micron accuracy of
photolithography used to define and align these pedestals and
standoff features, the application of this approach has proven to
be a viable alignment alternative. By effecting alignment in a
passive manner, the labor input into the finished product can be
reduced, resulting in lower cost of the final product.
One example of such an alignment scheme can be found in U.S. Pat.
No. 5,163,108 to Armiento, et al., the disclosure of which is
specifically incorporated herein by reference. The reference to
Armiento, et al. makes use of an alignment notch on the active
device which is designed to mate with alignment pedestals and
standoffs on the silicon waferboard. This particular structure is
used for aligning an optical fiber array to an array of light
emitting devices.
FIG. 1 is a perspective view of a laser array die 102 which is to
be mounted on a silicon substrate 100 such that the active region
106 of the laser die 102 accurately aligns with a fiber to be
placed in a v-groove 105 on the silicon substrate 100. As shown,
the die 102 has a notch 101 that has been etched therein to be an
accurately controlled distance from the laser active region 106.
Further, pedestals 103, 104, 108, and 109 have been fabricated on
the substrate at predetermined locations to serve as mechanical
fiducials for the laser die 102, i.e., the laser will be aligned by
virtue of contact with the fiducial. In particular, the laser die
is placed on the silicon substrate 100 generally in the vicinity of
fiducials 103, 104, 108, and 109 so that the active region 106
roughly aligns with the v-groove 105. The laser die 102 is then
pushed in the z direction so that the front surface 107 of the die
102 abuts mechanical fiducials 108 and 109 and in the x direction
so that the surface 112 of notch 101 abuts the surface 113 of
mechanical fiducial 103, thereby precisely aligning the laser die
102 on the silicon in the x and z directions in a position dictated
by the placement of the mechanical fiducials 103, 104, 109, and
110, (and notch 101).
Unfortunately, one problem with structures like the one shown in
the reference to Armiento, et al., is that it pertains only to
ridge laser structures. This is because, in a ridge waveguide laser
structure, the patterning photolithography step that defines the
active waveguide is simultaneously used to define the alignment
notch in the same mask level, resulting in an alignment of the
notch and active waveguide that is limited only by the variations
in the photolithography mask. However, it is advantageous from a
performance standpoint to be able to utilize lasers and other
active devices that incorporate a regrowth step, such as the buried
heterostructure laser described above. For this class of devices,
the subsequent regrowth step(s), bury the active waveguide mesa
and, hence, also the notch. Accordingly, fabrication is complicated
because the alignment notch must be made after the regrowth since
the notch patterning step must occur in a photolithography step
subsequent to the one in which the active waveguide is defined.
Moreover, a notch patterning step on the regrown surface of the
wafer is difficult because the mesa is not a visible re-alignment
feature using the conventional technique of optical alignment
methods. Even further, creating the notch using a different
photolithography step and mask than was used to create the mesa
increases the potential misalignment between the mesa and the
notch. Particularly, in such situations, the tolerances of the
masks are essentially cumulative. Further, additional error is
introduced by misalignment of the masks to one another.
Another known scheme for passively aligning an optoelectronic
device to an optical fiber on a silicon waferboard is the use of
visual fiducials and an optical detection system. In this
technique, visible markings (the fiducials) are made on the
surfaces of the optoelectronic device and mating markings are made
on the silicon waferboard. The visual fiducials usually are made by
etching through at least the outermost layer of the optoelectronic
device and the silicon waferboard to leave an aperture that can be
detected by an optical detection system. The fiducials are placed
on the optoelectronic device and the silicon waferboard in a
pattern so that when the optoelectronic device is positioned on the
silicon waferboard so that the fiducials on the optoelectronic
device perfectly overlay the mating fiducials on the silicon
waferboard, the two are properly aligned.
An optical detection system detects the visual fiducials on the
waferboard and optoelectronic device and then controls stepper
motors or equivalent means that align the optoelectronic device
with the waferboard so is that the fiducials properly mate with
each other, thereby resulting in proper alignment in at least two
dimensions of the optoelectronic device on the silicon waferboard.
The fiducial marks should be two dimensional, such as x's or
squares so as to provide visual reference cues in at least two
orthogonal directions. Further, typically two or more separate
fiducial marks that are spaced from each other are utilized on each
of the optoelectronic device and the waferboard. The use of two or
more spaced visual fiducials provide for greater accuracy in
alignment, particularly angular rotation about the y axis.
Particularly, the more individual fiducial marks and/or the further
apart they are from each other, that smaller will be any angular
rotation errors due to tolerance limits and the like.
FIGS. 2A through 2C illustrate the concept of visual fiducials.
FIG. 2A is a plan view of a silicon waferboard adapted to mate a
semiconductor laser to a fiber in a v-groove of a silicon
waferboard. FIG. 2B is a close up of the portion of the waferboard
surrounding the laser pad and FIG. 2C is a plan view of the laser
die. The waferboard 150 includes a laser bond pad 152 on which the
semiconductor laser 168 is to be mounted and a v-groove 154 within
which the fiber is to be placed. Also shown for sake of
completeness are a solder metalization 156 for a monitor pin behind
the laser and a wire bond pad 158 for electrically coupling the
laser to circuitry (not shown) on or off the silicon waferboard
150. Visual fiducials in the form of squares are shown at 160 and
162 on opposite sides of the laser pad 152. These squares actually
comprise holes etched through the various layers of the silicon
waferboard.
On laser die 168, strip 170 running longitudinally in the center of
the die is the active region of the laser. Mating fiducial marks
172 and 174 are etched in at least the outermost layer of the
semiconductor laser die. Visual fiducials 172 and 174 are shaped
and positioned to exactly mate with visual fiducials 160 and 162 on
the semiconductor die. All four fiducials are positioned such that,
when fiducial 172 exactly overlays fiducial 160 and fiducial 174
exactly overlays fiducial 162, active region 170 of the laser die
will precisely align with the core of a fiber placed in v-groove
154.
During assembly, the visual fiducial approach is implemented using
a bonding system that is capable of imaging the optical device
(particularly, fiducials 172 and 174) and the substrate fiducials
160 and 162. By suitable means, the optical device placement is
affected such that the optical device fiducials 172 and 174 lie at
a specified location relative to the substrate fiducials (typically
directly atop one another). Such a process can be performed either
automatically or manually. On completion of the device bond, the
active region 170 will precisely align with the core of a fiber
placed in the v-groove 154.
Obviously, the accuracy with which the active region 170 of the
laser die mates with the core of the fiber placed in v-groove 154
depends on many factors. For instance, it depends on the accuracy
of the bonding system used to place the die on the substrate. Also
of great importance is the placement accuracy of the visual
fiducials on the substrate wafer and on the laser die. This
placement accuracy, in turn, depends upon the accuracy of the
individual process masks used to define the visual fiducials and on
the number of process steps used to fabricate them. As the number
of process steps increases, fiducial location errors increase owing
to the accumulation of errors within the individual process masks
and to any alignment errors between the individual process
steps.
Another well known passive alignment technique involves the use of
metal pads formed on the surface of the silicon waferboard and
mating metal pads formed on the surface of the optoelectronic
device. Solder (or other wettable material such as brazing alloy,
Babbit metals, amalgams and even certain thermoplastic polymers) is
placed on the pads of either the waferboard or the optoelectronic
device die or both. Then, the optoelectronic device is roughly
aligned with and placed on the waferboard so that the mating pads
are in contact, where this initial placement need not be
particularly accurate. The solder is then reflowed (reheated to a
molten state) whereby final alignment is effected by the inherent
reduction in the surface tension within a liquid medium, the net
effect being to draw the mating metal pads into accurate alignment
with each other.
Just as discussed above in connection with the visual fiducial
technique for passive alignment, the accuracy of this technique is
highly dependent on the accuracy of the placement of the
metalization pads, which, again, is adversely affected as the
number of photolithography steps that are cumulatively relied upon
in aligning the metal pads relative to the active region of the
laser increases.
Accordingly, it is an object of the present invention to provide an
improved technique for passively aligning an optoelectronic device
with a fiber.
Accordingly, it is another object of the present invention to
provide an improved technique for passively aligning a buried
heterostructure laser with a fiber.
It is another object of the present invention to provide an
improved method and apparatus for aligning an optoelectronic device
on a mounting substrate, such as a silicon waferboard.
It is a further object of the present invention to provide an
improved method for creating mechanical fiducials, visual fiducials
and/or metalizations on an optoelectronic device.
SUMMARY OF THE INVENTION
The present invention relates to a technique for fabricating a
mechanical alignment fiducial or visual alignment fiducial on a
laser die particularly adapted for application with a laser die
that is a buried structure edge emitting laser. In fabricating the
device, a laser base structure consisting of (1) a buffer layer,
(2) a bulk or multiple quantum well (MQW) active layer, (3) a
cladding layer, and (4) a thin etch-stop alignment layer. A
dielectric, such as SiO.sub.2 or Si.sub.3 N.sub.4, is deposited
over the wafer. A photolithography patterning step, using
conventional techniques, is used to simultaneously define openings
in the SiO.sub.2 or Si.sub.3 N.sub.4 etching mask corresponding to
the mechanical alignment fiducial or visual fiducial and to the
laser waveguide mesa.
A reactive ion or other suitable etching technique is then used to
etch through the etch-stop alignment layer, cladding layer, and
active layer of the underlying structure to simultaneously define
the mechanical or visual fiducial and the laser waveguide mesa.
Photoresist is next applied and patterned such that the region in
the vicinity of the alignment notch is protected with photoresist,
while the active mesa region is exposed for a subsequent etching
step to further define the active mesa. After the final mesa
etching step, which utilizes an etchant which does not attack
photoresist, the remaining photoresist is removed, leaving a
structure which now incorporates a precisely located etch-stop
alignment layer that can serve as a mechanical or visual fiducial,
upon which additional regrowths may be performed using conventional
LPE or MOCVD techniques. This incorporated layer now functions as a
precise etching mask for material selective etchants, such as
HCl.
Next either a conventional unmasked LPE regrowth or a conventional
masked MOCVD regrowth step is performed to complete the structure.
The regrowth typically consists of several appropriately doped InP
layers, followed by a capping layer of, for example, InGaAs or
InGaAsP. A final patterning and selective etching step is used to
expose the etch-stop alignment layer bearing an accurately aligned
notch at the outer edge of this layer. The precisely located
surfaces of the etch-stop alignment layer can serve as either
mechanical or visual fiducials. For instance, the laser can be
positioned and pushed up against mating mechanical fiducials on a
silicon waferboard. Alternately, the fiducials can be made to have
two dimensional features and can be used as visual fiducials for
use in connection with an optical alignment technique. Even
further, the fiducials can be used as visual fiducials for precise
alignment of metal pads that are formed on the optoelectronic
device die, which metal pads will be used for later alignment of
the die on a substrate using solder re-flow or other similarly
principled techniques.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a prior art structure of a laser
die with a notch defining a mechanical fiducial and a substrate of
a silicon waferboard having corresponding mechanical fiducials.
FIG. 2A is a plan view of a substrate of a silicon waferboard with
visual fiducials.
FIG. 2B is a close up of a portion of FIG. 2A including a laser
mounting pad.
FIG. 2C is a plan view of a semiconductor laser die for mounting on
the silicon waferboard of FIGS. 2A and 2B.
FIG. 3 is a cross-sectional view of the invention of the present
disclosure showing the alignment fiducials on the silicon
waferboard making contact with the device to effect the passive
alignment by way of the alignment notch of the present
invention.
FIGS. 4-12 are cross sectional views of the various steps in
effecting the fabrication of the invention of the present
disclosure.
FIG. 13 is a cross sectional view of the buried heterostructure
laser with the alignment notch of the present disclosure.
FIGS. 14-16 show the processing steps to effect an alternative
embodiment of the invention of the present disclosure.
FIG. 17 is a cross-sectional view of an alternate embodiment of a
die in accordance with the present invention.
FIGS. 18 and 19 are cross-sectional views of a die at particular
stages in effecting the fabrication of the embodiment of the
invention shown in FIG. 17.
FIG. 20 is a cross-sectional view of a die in accordance with an
even further embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 3 is a cross sectional view of an optoelectronic device
fabricated in accordance with one embodiment of the present
invention and a substrate on which it is to be mounted. In this
example, the optoelectronic device is a buried heterostructure
semiconductor laser 200, the substrate is a silicon waferboard 250,
and the fiducial 205 is a mechanical fiducial. The silicon
waferboard includes silicon vertical standoffs 252 and side
pedestal 254 for aligning the laser 200 in the y and x directions,
respectively, using mechanical fiducial 205 on the laser.
To this end, n-type InP substrate 201 has a buffer layer 213 an
active quaternary layer 202, a cladding layer 203 and another
quaternary layer 204 that forms an etch-stop alignment layer as
discussed more fully below. A mechanical alignment fiducial in the
form of a notch 205 also is shown. The mesa structure is shown
generally at 206 with the InP blocking layers, which bury the mesa
laterally, shown at 207 as well as blocking layers 208 which
provide for current confinement so that current is prohibited from
passing between the mesa and the side Q-active layer. A p-type
burying layer is shown at 209 with the p-type contact layer at 210.
The standoffs 252 contact the top surface 211 of the etch-stop
alignment layer 204 for registration (alignment) in the y-direction
(reference the coordinate axis shown). A second surface or a side
surface 212 effects the alignment to a side pedestal 254 for
registration (alignment) in the x-direction.
To be very clear, there are two surfaces which are used for
alignment. The first surface is for height registration to
standoffs 252, and is shown at 211. It is the top of the quaternary
layer 204. A second surface is shown at 212 for registration in the
x-direction. In the embodiment shown in FIG. 3, side surface 212
runs through quaternary layer 204, cladding layer 203, active layer
202 and buffer layer 213. It will be understood by those of skill
in the art that the term notch is commonly used in the industry to
denote apertures in layers of semiconductors. Therefore notches may
have one or more vertical walls. For instance, a notch with a
square cross-section (in a horizontal plane) would be considered to
have four perpendicular surfaces (or walls). Since notch 205 in
FIG. 3 happens to extend to the periphery of the die, it has only
one side, i.e., side surface 212. However, a notch in accordance
with the present invention may have any number of sides. If the
fiducial will be used to achieve alignment mechanically (i.e, by
contact) in only one direction, then one wall is adequate. If the
fiducial is to be used to achieve alignment mechanically in two
dimensions, then two walls is adequate.
Both the height registration (y-axis in FIG. 3) as well as the side
registration to a pedestal 254 (x axis in FIG. 3) are effected very
precisely by the invention of the present disclosure. To this end,
the distance from the center of the mesa 206 to the side surface
212 is toleranced very tightly through the precision of
photolithographic alignment and etching techniques. This is shown
as a distance d in FIG. 3. Furthermore, the height registration
which accurately aligns the active layer 202 of mesa 206, where
light emission occurs, to an optical fiber in the y-direction is
tightly controlled by the thickness between the active layer and
the top surface 211 of the etch-stop alignment layer 204.
It is of particular importance to note that the etch-stop alignment
layer in the present invention, through the use of selective
etching, allows for the precision in the x-direction and, through
precision growth techniques, allows for the precision alignment in
the y-direction. Since the active region 202 of mesa 206 is defined
by the same mask that defines the notch 205, namely, the mask used
to create the windows through the etch-stop layer 204, any
misalignment between the mechanical fiducial (notch 205, including
surface 212) and the active region 202 of the mesa 206, which is
the feature that ultimately is being aligned (with an optical
fiber, for instance), is limited to the tolerance of a single mask.
This tolerance is on the order of about 0.2 microns for many of the
maskwork techniques in wide use today. Furthermore, a mask to mask
alignment tolerance of about 0.1 microns is typical. As the number
of masks between the mask used to define the feature that is
ultimately being aligned and the mask used to define another
feature relied upon for making that alignment increases, the
accuracy of the alignment decreases accordingly. For example, if a
fiducial relied upon in the alignment of the active region of a
laser to a fiber was created by a different mask than the mask that
defined the active region, then the tolerance of the alignment will
be 0.2+0.2+0.1=0.5 microns (worst case). In this example, if the
mask that defined the fiducial was aligned based upon a third mask,
the alignment of which was based upon the mask that defined the
active region, then the alignment tolerance would be
0.2+0.2+0.2+0.1+0.1=0.8 microns (worst case).
In the present invention, the etch-stop alignment layer 204 defines
both the active region 202 of the mesa 206 and the notch side
surface 212 with the same mask while allowing the notch 205 to be
exposed even after regrowth of the InP blocking and burying layers
that are used in the fabrication of a buried heterostructure so
that the notch can, in fact, be used as a mechanical or a visual
fiducial.
A fabrication process in accordance with the present invention is
illustrated by FIGS. 4-20, with various embodiments disclosed.
FIGS. 4-16 disclose embodiments in which the invention employed to
create a notch such as notch 205 with edge 212, which can be used
as a mechanical fiducial as described above in connection with the
Armiento patent.
Turning to FIG. 4, the InP substrate 201, has disposed thereon an
InP buffer layer 213, an InGaAsP bulk or multiple quantum well
(MQW) quaternary active layer 202, an InP cladding layer 203, and a
thin quaternary InGaAsP etch-stop alignment layer 204. A dielectric
layer 305 such as SiO.sub.2 or Si.sub.3 N.sub.4 is deposited over
the wafer using standard technique. A photolithography patterning
step using conventional techniques is used to define openings in
the SiO.sub.2 or Si.sub.3 N.sub.4 to form an etching mask
corresponding to the alignment notch edge 212 and the laser
waveguide mesa 206.
Turning to FIG. 5, a reactive ion or other suitable etching
technique is then used to etch through the InGaAsP etch-stop
alignment layer 204, the InP cladding layer 203, and the active
layer 202 of the underlying structure to define the laser waveguide
mesa 206 and the alignment notch edge 212. Turning to FIG. 6,
photoresist 506 is next applied and patterned such that the region
of the vicinity of the alignment notch is protected with
photoresist, while the active mesa region is exposed for a
subsequent etching step to further define the active mesa, as seen
in FIG. 7. After the final mesa etching step, which utilizes an
etchant which does not attack photoresist, the remaining
photoresist is removed, leaving a structure which now incorporates
a precisely located InGaAsP etch-stop alignment layer 204 upon
which additional InP regrowths may be performed using conventional
LPE or MOCVD techniques. This incorporated layer now functions as a
precise etching mask for material selective etchants, such as HCl.
Thereafter, as shown in FIG. 8, a layer of blocking material 207,
InP, is deposited by metal-organic chemical vapor deposition
(MOCVD) or grown by liquid phase epitaxy (LPE).
If MOCVD is used, a dielectric mask is utilized as is shown at 709
in FIG. 8. With the dielectric 709 as the mask, the MOCVD technique
produces a nearly planarized InP blocking layer. The InP blocking
layers may contain p-type InP, for example, if zinc is used as a
dopant; n-type InP, for example, if sulfur is used as the dopant;
and/or semi-insulating InP, for example, if the dopant is iron. To
this end, multiple combinations of any of these three types of InP
can, in fact, be used. These are shown as the blocking layers, and
are effected in the first regrowth. By using a photoresist to
protect the dielectric in the central portion of the chip, shown at
710, the dielectric shown at 709 on the sides of the chip are
etched selectively from the surface. The resulting structure is as
shown in FIG. 9 with the dielectric material shown at 710, the
substrate at 201, the InP buffer layer at 306, the active layer at
202, the cladding layer at 203, the etch-stop alignment layer at
204, and the InP blocking layer at 207 and which correspond to the
various layers in previous drawing figures.
After the first blocking layer regrowth is complete, a second InP
blocking layer 208 is deposited. In the present discussion, MOCVD
is utilized to effect this deposition. In addition, as before, the
second InP blocking layer can be p-type, n-type or intrinsic InP or
any multiple combinations of these three types of InP. This second
layer of blocking material is used to block current flow between
the side sections shown at 915 in FIG. 10 and the central mesa
shown at 206. Furthermore, after the dielectric layer which covers
the mesa 206 is removed, the p-type InP burying layer is deposited
by MOCVD or grown by LPE techniques. Either of these techniques can
thereafter be used to effect the growth or deposition of the
InGaAsP p-contact layer.
Referring to FIG. 11, the p-type burying layer is shown at 209 and
the p-type contact layer is shown as 1016 in FIG. 11. Finally, in
order to reveal the alignment notch for effecting the alignment of
the notch with the silicon vertical standoffs 252 and side pedestal
254 on a silicon waferboard 250 (FIG. 3), the contact layer 1016 in
FIG. 11 is partially etched as shown at 210 in FIG. 12. Using a
solution of hydrochloric acid, for example, hydrochloric acid and
water or a mixture of hydrochloric acid and phosphoric acid, the
etch-stop alignment layer 204 is revealed by etching the InP
burying layer and the InP blocking layers. This is shown in FIG. 13
with the etching revealing notch 205, including surfaces 211 and
212. The first surface 211 is for alignment in the y direction to
standoffs (e.g., 252 in FIG. 3), while the side surface 212 of
notch 212 is for registration in the x-direction to a side pedestal
(e.g., 254 in FIG. 3). In this etching step to reveal surface 211
and the notch 205, selective etching is used whereby the InP is
etched. To this end, the InP burying layer shown at 209, the second
regrowth InP layer shown at 208 and the side layer of InP 207
effected in the first regrowth are selectively etched. This reveals
the first surface 211, which receives the vertical standoffs 252
for y-direction registration as is shown in FIG. 13. Additionally,
the notch along the side referenced as 205 in FIG. 13 is effected
in this etch step. The side surface 212 is used for the
registration in the x-direction to a side pedestal 254.
In the technique where LPE is used to grow the InP layers, the
basic techniques used up to the step shown in FIG. 7 are carried
out. With the structure shown in FIG. 7, the photoresist and
dielectric, shown as 506 and 505, respectively, are removed. LPE is
thereafter used to regrow p-type InP blocking layer, the n-type InP
blocking layer and the p-type InP burying layer. Additionally, the
p-type contact layers of InGaAs are also grown by LPE. The final
structure after LPE is used to grow InP as well as the contact
layers is shown in FIG. 14. Thereafter, a portion of the InGaAs
contact layer is removed as shown at 210 in FIG. 15. Thereafter,
using a solution of HCI which could be, again, HCl and water or HCl
and H3PO4, the Q-alignment layers are revealed. This is shown in
FIG. 16. Again, as shown in FIG. 16, a first surface 211 is
revealed and enables the alignment in the y-direction as is shown
in FIG. 16. Furthermore, as shown in FIG. 16, the side surface 212
and the alignment notch 205 as well as the region 210 are revealed
through this second etching step which etches the InP, but does not
etch the InGaAsP layers.
By virtue of the fact that the active and etch-stop alignment
layers are defined at the same time by very precise
photolithographic patterning and etching techniques to sub-micron
accuracy, the accuracy in the x- and y-directions achieved by the
present invention allows the edge emitting buried laser chips to be
very accurately passively aligned to a single mode optical fiber,
disposed on a silicon waferboard.
While FIGS. 3-16 illustrate embodiments of the invention in which
the fiducial being created is a mechanical fiducial, namely notch
205 with side surface 212, that can be used for passive alignment
in accordance with the method described and shown in connection
with FIG. 1, the invention is not limited to such application. The
invention also can be applied to define visual fiducials such as
two dimensional apertures such as the squares illustrated in FIG.
2C. Those squares can then be used for optical alignment as
described in connection with FIGS. 2A-2C. For instance, the
fabrication steps outlined in connection with FIGS. 3-16 could be
followed essentially as previously described, with the only
significant difference being that, the appropriate etch masks would
define a square having four side walls, rather than merely a notch
having only one side wall 212.
FIG. 17 is a cross-sectional view of a laser die 1700 in accordance
with this type of embodiment of the invention. FIG. 17 corresponds
to FIG. 2 of the first disclosed embodiment of the invention. The
laser die 1700 is essentially identical to the laser die 200 in
FIG. 3 except that, instead of providing merely a notch 205 with a
single side wall 212, the appropriate etch mask that defined the
notch 205 in the FIG. 3 embodiment is replaced with a mask that
instead defines a square void 1701 that is enclosed on all lateral
sides by four lateral walls. Three of the walls, 1712, 1713, and
1714, are shown in the Figure. Since the Figure is a cross-section
of the die, the fourth wall, which opposed wall 1713, cannot be
seen. Also, in a practical embodiment, there would be at least a
second void on the opposite side of the mesa from void 1701.
More specifically, referring to FIGS. 4, 5 and 18, the mask used in
the photolithography step described in connection with FIG. 4 to
create the openings in dielectric layer 305 would be replaced with
a mask that defines opening 1801 as square, rather than merely
allowing everything to the edge of the die to be etched (as in FIG.
4). Thereafter, the etching step described in connection with FIG.
5 is performed essentially as described in connection is with FIG.
5. However, because the pattern in dielectric layer 305 is
different, it creates the intermediate structure shown in FIG. 19
with void 1701, rather than the intermediate structure shown in
FIG. 5. All subsequent processing steps as described in connection
with FIGS. 5-16 remain essentially the same. The result is a two
dimensional visual fiducial created in the same photolithography
step that created the mesa and which can be used for optically
aligning the laser die with corresponding visual fiducials created
on a substrate, as described in connection with FIGS. 2A-2C.
The process of the present invention can further be adapted to
embodiments in which metal pads are fabricated on the laser die for
use in a solder reflow or similar surface tension alignment
technique as described above. Specifically, the metal pads formed
on the laser die must themselves be properly positioned relative to
the mesa in order for the surface tension alignment technique to
work. Extremely accurate alignment of the metal pads to the mesa
can be achieved by using a structure created in accordance with the
techniques described in this specification as a visual fiducial for
aligning the mask(s) that defines the metal pads. In this manner,
the metal pads will be aligned with the mesa with minimal error
since they are aligned directly to a structure that was defined in
the same photolithography step that defined the mesa. For instance,
the visual fiducials fabricated as set forth above in connection
with FIGS. 17-19 could just as easily have been used as visual
fiducials for aligning the masks for defining the metal pads as
they could for direct optical alignment of the laser die on the
substrate.
Even greater accuracy can be achieved in connection with surface
tension alignment techniques by forming the metal pads and/or
depositing solder directly in the voids created as described above
in connection with FIGS. 17-19. FIG. 20 illustrates such an
embodiment. In particular, void 1701 is filled with a deep metal
pad 2001 which extends all the way from the top of layer 306,
through layers 202, 203 and 204, to the top surface of layer 204.
Then solder or other wettable material 2003 has been placed on top
of the metal pad 2001. In alternate embodiments, the solder 2003 on
the voids of the die may be omitted and instead be placed on the
metal pads of the substrate onto which the die is to be mounted. In
even further embodiments, solder or other wettable material might
be placed both on the die voids and the substrate pads. The solder
essentially would be positioned extremely accurately relative to
the mesa since the mask that defined the mesa is the same mask that
defined the position of the pads. Of course, there would be a
separate mask used in the actual creation of the solder and/or
metal pads in order to assure that the metal or solder is deposited
only in the apertures. Nevertheless, the mask that defined the mesa
and apertures would play a substantial role in defining the
positions of the apertures.
Having thus described a few particular embodiments of the
invention, various alterations, modifications, and improvements
will readily occur to those skilled in the art. Such alterations,
modifications and improvements as are made obvious by this
disclosure are intended to be part of this description though not
expressly stated herein, and are intended to be within the spirit
and scope of the invention. Accordingly, the foregoing description
is by way of example only, and not limiting. The invention is
limited only as defined in the following claims and equivalents
thereto.
* * * * *